An emulsion is a dispersion of one liquid in another liquid and generally is in the form of a water-in-oil mixture having an aqueous or water phase dispersed within a substantially immiscible continuous oil phase. Water-in-oil (or oil in water) emulsions having a high ratio of dispersed aqueous phase to continuous oil phase are known in the art as High Internal Phase Emulsions, also referred to as “HIPE” or HIPEs. At relatively high dispersed aqueous phase to continuous oil phase ratios the continuous oil phase becomes essentially a thin film separating and coating the droplet-like structures of the internal, dispersed aqueous phase. In certain HIPEs continuous oil phase comprises one or more polymerizable monomers. These monomers can be polymerized, forming a cellular structure, for example a foam, having a cell size distribution defined by the size distribution of the dispersed, aqueous phase droplets.
HIPEs can be formed in a continuous process, wherein a HIPE is formed and then moved through the various stages used to produce a HIPE foam. A movable support member, such as a belt will typically be used to move a HIPE from one stage to the next. The initial polymerization of a HIPE comprises the initial 10-20% polymerization of the monomers present in the oil phase. Following the initial polymerization of the HIPE the next stage involves the bulk polymerization of the monomers present in the oil phase to produce a HIPE foam. The bulk polymerization stage lasts until 85 to 95% of the monomer has peen polymerized into a HIPE foam.
Initiator, which is used to start polymerization, is generally added during HIPE formation either to the separate aqueous and continuous oil phases or to the HIPE during the emulsion making process. In addition to the presence of initiator heat can be used to accelerate the polymerization reaction, for example the individual aqueous and oil phases may be heated to accelerate the polymerization reaction.
In a continuous process following HIPE formation, a HIPE can be moved to a curing oven, to complete polymerization. One type of curing oven has multiple levels or tiers with each tier having a belt running in the opposite direction from the belt above or below it. These multi-tiered curing ovens provide an enclosed heating environment and a large belt surface area in which to polymerize the HIPE monomers. Further, multi-tiered curing ovens take up very little floor space compared to horizontally designed curing ovens and are economic to run, in that the total volume is relatively small compared to the belt surface area so less energy has to be expended to heat the oven. The multiple tiers of the curing oven allow the top and bottom surfaces of a HIPE to be reversed from one level to another, such that as the HIPE progresses downwardly from tier to tier through the curing oven, the top surface of the HIPE (which is not in contact with belt surface) will be reversed on the belt of the next downward tier, so that the formerly top surface of the HIPE is now in contact with the belt at the next tier, such that the top surface of the HIPE is now the bottom surface. When the HIPE reaches the next level in the curing oven the HIPE surfaces will be reversed again. However, as the HIPE is not fully polymerized when it enters the multi-tiered curing oven the first couple of times the HIPE surfaces are reversed, parts of the HIPE surface that were in contact with the belt surface may adhere to the belt surface. These adherents on the belt surface then cause harmful defects, such as discoloration and reduction in the structural integrity, of the HIPE that subsequently come in to contact with the belt surface.
One potential solution to the problem of HIPEs adhering to the belt surface has been to increase the level of initiator in the HIPE or the temperature at which the HIPE is formed. However, both of these potential “fixes” have several drawbacks. First, both accelerate polymerization of the HIPE before the HIPE has been deposited on a belt, as initiator must be added before the HIPE is deposited so the HIPE will maintain some form upon contact with the belt, so as not to uncontrollably spread or cause deformations in the HIPE. The end result of accelerated polymerization during HIPE formation, either by additional initiator or heat, is that the polymerized portions of the HIPE clog whatever device is used to deposit the HIPE on to a belt, such as a die, leading to increased down time and increased costs. HIPEs can be polymerized in a continuous fashion by several different polymerization methods; such as thermal polymerization, radiation induced polymerization, and redox induced polymerization. While these methods can be used to polymerize HIPEs, there are limitations to their usefulness in all of the stages of HIPE polymerization. For example, HIPEs undergoing thermal polymerization are not optimized for an initial polymerization stage because they must be stable in the mixing process to avoid pre-polymerization in the mixing equipment, as such additional time is required for the initial polymerization.
Therefore, a method is needed is needed to prevent a HIPE surface from adhering to the belts of a multi-tiered curing oven, but which does not adversely affect other stages of HIPE formation.